The present invention relates to the use of parabolic reflectors in an antenna system for use in broadband satellite communications. More specifically, the invention relates to an antenna array of parabolic rectangular reflectors having antenna feeds which are offset in order to reduce antenna sidelobe levels.
In the field of satellite communications, antenna systems for satellite communication are required to have a broad bandwidth while having a narrow antenna beam width. The broad bandwidth enables the antenna system to both transmit and receive signals over frequency bands of several GHz. The narrow antenna beam width provides a high gain for signals that are received and transmitted over a particular frequency to and from a particular satellite, and provides discrimination between satellites.
Although the antenna beam width is usually focussed on a particular satellite, it may also be necessary to alter the focus of the antenna beam toward another satellite.
Due to the high speed at which aircraft travel, antenna systems which are mounted on aircraft are required to maintain a low profile. The low profile minimizes drag. Typically, an antenna system is placed within a radome that has a height restriction in the range of 4 inches to 12 inches depending on the type of aircraft.
Single parabolic reflectors are not ideal for use in applications requiring a low profile. This is due in part to the fact that a parabolic reflector has a low aspect ratioxe2x80x94it is difficult to optimally illuminate the entire reflector surface when the ratio of the aperture width to height is large. In order to illuminate the entire surface of the parabolic reflector, the reflector itself must be distanced from the reflector feed. For example, a parabolic reflector having a surface width of 28 inches would typically require the feed to be placed at least 10 inches from the reflector. This is well beyond the height restriction of the radome on an aircraft. Regardless of whether the feed is axial or offset, inside the radome, the geometry of a single parabolic reflector is less than ideal for use on an aircraft fuselage.
The use of contiguously disposed parabolic reflectors produces a high gain and a narrow central beamwidth. However, two large sidelobes are producedxe2x80x94one on either side of the antenna beam peak. The sidelobes are introduced due to the modulation of the aperture illumination resultant from the radiation pattern of the antenna feeds. Techniques are required to minimize the impact of modulation, resulting from the aperture illumination, and provide lower sidelobes on either side of the main antenna beam when utilizing an array of contiguously disposed parabolic reflectors.
U.S. Pat. No. 6,049,312, issued to Lord, discloses an antenna system with a plurality of reflectors for generating a plurality of beams. Lord teaches an antenna system comprising a first reflector and a second reflector, as well as corresponding first and second feeds. While the two feeds are offset from their respective reflectors, the first and the second reflector are in a substantially tandem arrangement and not contiguously disposed in array. Rather, Lord teaches a compact antenna configuration whereby the first reflector and the first feed cooperate to form a first antenna beam and the second reflector and the second feed form a second beam. Lord does not discuss the formation of a main antenna beam in which the antenna sidelobe levels may be reduced by displacing the feeds and the foci of the respective reflectors.
U.S. Pat. No. 6,262,689, issued to Yamamoto, discloses an antenna system for communicating with low earth orbit satellites from the ground. In one embodiment, Yamamoto teaches the use of two reflectors separated by a predetermined distance, each reflector having a primary feed for radiating a beam onto its respective reflector, and a switching means to switch the antenna focus between various satellites. However, Yamamoto teaches the tracking of two satellites, one by each of the reflector/feed systems. The Yamamoto patent does not disclose an antenna system which reduces the sidelobe level of the antenna beam.
In view of the above shortcomings of the prior art, the present invention seeks to provide an array of two antenna elements, wherein each antenna element has a feed that is displaced toward the center of the antenna array. Furthermore, the present invention seeks to provide an antenna system utilizing feedhorns, parabolic reflectors, a common aperture surface, and several pairs of contiguously disposed reflectors having displaced feeds to reduce antenna sidelobe levels. Moreover, the present invention seeks to provide an antenna array of parabolic reflectors with lower sidelobes adjacent to the main antenna beam.
The present invention is an antenna array of parabolic rectangular reflectors for use in satellite communications. The antenna comprises two parabolic reflectors disposed contiguously on a common outer surface. The common surface forms a continuous antenna aperture. The parabolic reflectors have rectangular side edges which permit the adjacent edges of the parabolic reflectors to be spaced closely. The mouth of each parabolic reflector is focussed on a separate feed. The focus of the feed is not located at the center of the reflector but rather offset. The antenna feeds and the reflector foci are displaced toward the center of the array such that the spacing between the antenna feeds is less than half the length of the antenna. The present invention provides the displacement of each reflector focal point and each antenna feed toward the center of the array.
According to the present invention, the antenna feeds are excited coherently in order to produce a narrow well focussed beam. Support struts, located between the feeds and their respective parabolic reflector, are designed such that they minimize the blockage of the antenna aperture. In one embodiment, the antenna array may be mounted on the fuselage of an aircraft. The antenna is steered mechanically in elevation and azimuth to maintain the antenna attitude directed toward a particular satellite at all times. Finally, the displacement of the antenna feeds and reflector foci result in lower sidelobes adjacent to the main antenna beam.